The formation of a blood clot and its successive dissolution, referred to as the hemostatic process, is required to arrest blood loss from an injured vessel. This process is the result of a delicate functional balance between plasma coagulation factors, platelets, and fibrinolytic proteins. Each of these elements plays an important role in activating/deactivating the others, and the appropriate stimuli are necessary to prevent excessive blood loss without causing inappropriate thrombosis. Disruption of this balance plays a significant role in the onset of potentially fatal conditions, including myocardial infarction, stroke, deep vein thrombosis, pulmonary embolism, and hemorrhage.
The hemostatic process is initiated by the activation and subsequent adhesion of platelets to the site of injury within the vessel wall. Activated platelets recruit other platelets and interact with fibrinogen in the blood plasma to form a platelet-plug that serves as the initial response to stop blood loss. Hemostasis then proceeds with a cascade of proteolytic reactions of the plasma coagulation proteins that ultimately form a three-dimensional network of fibrin that strengthens the platelet-plug. The fibrin chains are cross-linked and stabilized by the plasma factor XIIIa (FXIIIa). Platelets also have a central role in regulating the process of fibrin polymerization. The final step of hemostasis (i.e., fibrinolysis or clot dissolution) involves the activation of the plasma protein plasmin, which lyses the blood clot when its useful life is over. This cell-based model of hemostasis closely reflects the in vivo physiological process.